The referenced U.S. Pat. No. 4,808,887, Fahnrich and Zuchtriegel, the disclosure of which is hereby incorporated by reference, describes a circuit which includes an active harmonic filter operating in a double pump system. The active harmonic filter has a capacitor which is coupled to a center terminal between diodes of the double pump filter and a center terminal between two transistors which form the high-frequency oscillator circuit providing high frequency energy to the fluorescent lamp.
The circuit, with the filter as described in the referenced patent, ensures sufficient filtering so that the standards required by the International Electrotechnical Commission, IEC Standard 555- 2 of 1982, and amended by an amendment No. 2 of Sep. 1988 (which incorporates a prior amendment No. 1 of Oct. 1985), is met. The circuit functions as described and economically meets the IEC standards and specifications only, however, if the lamp power does not exceed 35 W. If lamps with higher lamp power are to be connected in such a circuit, or a plurality of lamps are connected together, either in parallel or in series, it has been found that the pumping capacitors must have capacity values of substantial level so that, in a push-pull amplifier, substantial switching losses will result. Thus, although the circuit functions well, the circuit losses in connection with its use render it uneconomical. Further, the network current may have a wave shape which, at the zero cross-over, has higher harmonic contents than permissible by the IEC standard, resulting in suddenly changing wave slopes rather than in essentially pure sine functions.